Mike, Maybe change-clefs should not get the full extra-spacing-height by
default.
Engravers tend to tuck change-clefs in with the least possible disruption of
the note spacing.
Also, the test 'dots.ly' shows a ledger line sliding under a bar line.
I think the (-1 . +1) extra-spacing height for the BarLine is not quite enough.
LilyPond gives ledger line columns only the minimum necessary extra-spacing
height, which I think is correct because traditionally and practically people
slide neighboring accidentals under ledger lines.
Also, there are still two definitions for extra-spacing height in TimeSignature.